Voters in Kaduna and Bauchi states will now go to the polls on April 28, said Attahiru M. Jega, chairman of the Independent National Electoral Commission. Elections will continue April 26 in other states.

"This is to allow for further cooling of tempers and for the security situation in those states to improve," Jega said.

The development came after Goodluck Jonathan, who was declared the winner in the recent presidential election, warned perpetrators of postelection violence that "enough is enough. He said the unrest in a series of elections brings back memories of events leading up to a 1960s civil war in the nation.

Jonathan, who ran as the incumbent, hails from the oil-rich southern region, which is majority Christian.

Parts of the mostly Muslim north charged the elections were rigged. Armed protesters in the region prowled the streets chanting the name of former military ruler Muhammadu Buhari, the main opposition frontrunner.

"We are shocked by these horrific acts, which strike at the heart of our nation," Jonathan said in a statement Thursday. "These acts of mayhem are sad reminders of the events which plunged our country into 30 months of an unfortunate civil war."

Muslims fearing reprisal attacks in Christian areas have fled to military barracks or neighboring states, the Nigerian Red Cross said.

More than 40,000 people are displaced in the north, according to the aid group. Government officials have declined to release the number of fatalities or injuries over fears it could prompt revenge attacks.

"There are ... no grievances that our laws and courts cannot address," he said.

Jonathan's warning comes two days after he pledged a "new dawn" for the nation following the vote hailed as a significant improvement from past polls.

The last election four years ago was widely condemned for rampant vote rigging, violence, theft of ballot boxes and intimidation. While Saturday's election was marred by violence, reports of underage voting and other logistical problems, observers considered it an improvement.

Action Congress of Nigeria, an opposition party, Thursday called the presidential election "the most systematically rigged election in Nigeria's history" and warned against a repeat in next week's gubernatorial elections.

''President Jonathan should call his dogs of war at the PDP (Peoples Democratic Party) to order, so that they will not push their luck by attempting to thwart the will of the people any more than they have already done, with the grim consequences that we all can now see,'' the party said.

World leaders have urged the west African nation to probe allegations of ballot stuffing and unusually high turnout in some areas.

Nigeria is Africa's largest oil producer and its most populous nation with 150 million people.